1. Field of the Invention
The inventon is directed to a grapple having two double acting hydraulic cylinders, one for each tong. A closed center flow divider and a cross over relief valve assembly are hydraulically positioned between the double acting hydraulic cylinders.
2. Description of the Prior Art
Skidders are large machines used for moving logs in a forest setting. Some skidders are provided with grapples which are used to grasp and hold the logs during transport. The grapples are provided with a grapple head which is mounted to the skidder. The grapple head is provided with two downwardly depending tongs for grabbing the logs. Some grapples use a single double acting hydraulic cylinder to grab and hold the logs, see U.S. Pat. Nos. 4,396,216, 4,431,365, and 4,576,406. The geometry of a single cylinder grapple is arranged so that the tongs moved together.
Grapples having two double acting hydraulic cylinders have also been used. A first hydraulic cylinder is used to drive the right tong and a second hydraulic cylinder is used to drive the left tong, see U.S. Pat. Nos. 4,023,848, 4,452,479, and 4,542,929. Typically these units are provided with a standard spool type flow divider or no flow divider. If no flow divider is used the tongs are subject to gross mistiming when higher resistance is encountered by one of the tongs. This is desirable when a non-uniform load is grappled because an equal grappling force is applied by both cylinders. A flow divider helps to maintain timing, but retiming is slow as it can only be made at the stalled leakage flow rate of the divider. In addition when the control valve is in neutral, the flow divider is in an open center position and application of an external load can cause flow between the cylinders resulting in the collapse of one tong and the opening of the other tong.